Fertilizer knives are employed by farmers to deliver nitrogen enriching fertilizers such as anhydrous ammonia or liquid fertilizers to a targeted zone beneath the surface of the soil of a crop field while causing a minimum of disruption to the structure of the upper layer of the topsoil. Typically, a fertilizer implement carries a pattern of fertilizer knives spaced for simultaneously delivering fertilizer to a plurality of parallel furrows.
Typical prior art fertilizer knives are cast as one piece or are more commonly a combination of a tough steel knife blank and cast iron knife point that is welded to the knife blank. The cast iron knife point has a wedge-shaped leading edge for cutting through soil. With a typical prior art fertilizer knife, a steel fertilizer tube is welded to the trailing edge of the knife blank. For most fertilizer knives, the steel fertilizer tube is welded to the back surface of the knife blank and the fertilizer tube terminates at the base of the knife. Because the fertilizer knife is usually fashioned from a combination of steel and cast iron, the fertilizer tube is fashioned from steel so it can be welded to the trailing portion of the fertilizer knife. The use of a steel fertilizer knife tube presents two disadvantages. First, the skilled reader may recall that the knife shank is bolted to an implement shank with two bolts—an upper bolt and a lower bolt. The lower bolt is designed to function as a structural fuse. If the fertilizer knife encounters an obstacle during use, the lower bolt fails and allows the fertilizer knife to swing back without damaging the fertilizer implement. However, when the lower attachment bolt fails, and, as the fertilizer knife swings back, the steel fertilizer knife tube is often damaged beyond repair. While this action has protected the implement from damage, a damaged fertilizer knife tube makes it necessary to replace the knife. Although an operator will often have spare shank bolts on hand, an operator will usually not have a replacement fertilizer knife available. Accordingly, the damaged fertilizer tube often causes a delay in operations. Second, in the case of anhydrous ammonia, NH3, which boils at approximately −30° F., it is highly advantageous to deliver NH3 to the soil in liquid form. This is because the liquid NH3 absorbs large amounts of heat when it boils. Further, vaporization of NH3 in the highly thermally conductive steel fertilizer knife tube causes great amounts of heat to be transferred into the fertilizer knife tube from the steel and cast iron knife. This is because the heat of vaporization of liquid NH3 is far greater than its specific heat. In relatively cool soil conditions, this can cause ice to accumulate at the lower end of the fertilizer knife which greatly reduces the utility and function of the fertilizer knife. What is needed is a fertilizer knife that eliminates the above noted disadvantages inherent in a fertilizer knife having a steel fertilizer tube.